My spikes don't damage the player

Questions & Answers
1

This show this error: NullReferenceException: Object reference not set to an instance of an object
Spikes.Start () (at Assets/Spikes.cs:12)
The scripts are:

Spike script :

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Spikes : MonoBehaviour
{

    private PlayerController2D player;

    void Start()
    {
        player = GameObject.FindGameObjectWithTag("Player").GetComponent<PlayerController2D>();
    }

    void OnTriggerEnter2D(Collider2D col)
    {

        if (col.CompareTag("Player"))
        {
            player.Damage(1);
        }
    }
}

PlayerController2D script :

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.SceneManagement;

public class PlayerController2D : MonoBehaviour
{
    Animator animator;
    Rigidbody2D rb2d;
    SpriteRenderer SpriteRenderer;

    bool isGrounded;

    [SerializeField]
    Transform groundCheck;
    [SerializeField]
    Transform groundCheckL;
    [SerializeField]
    Transform groundCheckR;

    [SerializeField]
    private float walkSpeed = 1.5f;

    [SerializeField]
    private float jumpSpeed = 5f;

    //Stats
    public int curHealth;
    public int maxHealth = 100;

    // Start is called before the first frame update
    void Start()
    {
        animator = GetComponent<Animator>();
        rb2d = GetComponent<Rigidbody2D>();
        SpriteRenderer = GetComponent<SpriteRenderer>();

        curHealth = maxHealth;
        
    }

    private void Update()
    {
        if(curHealth > maxHealth)
        {
            curHealth = maxHealth;
        }
        if(curHealth <= 0)
        {
            Die();
        }
    }



    private void FixedUpdate()
    {
        if((Physics2D.Linecast(transform.position, groundCheck.position, 1 << LayerMask.NameToLayer("Ground"))) ||
          (Physics2D.Linecast(transform.position, groundCheckL.position, 1 << LayerMask.NameToLayer("Ground"))) ||
          (Physics2D.Linecast(transform.position, groundCheckR.position, 1 << LayerMask.NameToLayer("Ground"))))

        {
            isGrounded = true;
        }
        else
        {
            isGrounded = false;
            animator.Play("Player_jump");
        }

        if(Input.GetKey("d") || Input.GetKey("right"))
        {
            rb2d.velocity = new Vector2(walkSpeed, rb2d.velocity.y);
            
            if(isGrounded)
                animator.Play("Player_walk");

            SpriteRenderer.flipX = false;
        }
        else if(Input.GetKey("a") || Input.GetKey("left"))
        {
            rb2d.velocity = new Vector2(-walkSpeed, rb2d.velocity.y);

            if (isGrounded)
                animator.Play("Player_walk");

            SpriteRenderer.flipX = true;
        }
        else
        {
            if (isGrounded)
                animator.Play("Player_idle");

            rb2d.velocity = new Vector2(0, rb2d.velocity.y);
        }

        if(Input.GetKey("space") && isGrounded)
        {
            rb2d.velocity = new Vector2(rb2d.velocity.x, jumpSpeed);
            animator.Play("Player_jump");
        }
    }

    void Die() 
    {
        //Restart
        SceneManager.LoadScene(SceneManager.GetActiveScene ().buildIndex);
    }

    public void Damage(int dmg)
    {
        curHealth -= dmg;
    }

}

HUD Script:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class HUD : MonoBehaviour
{
    public Sprite[] HeartSprites;

    public Image HeartUI;

    public PlayerController2D player;

    void Start()
    {
        
    }

    void Update()
    {
        HeartUI.sprite = HeartSprites[player.curHealth];
    }
}
2

Did you make sure to add the “Player” tag to your player with the PlayerController2D script?

3

the player/playercontroller probably didn’t spawn when the start void was called, if you change it in the inspector you would have to do that every time so i surgest this:
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Spikes : MonoBehaviour
{

    private PlayerController2D player;

    public float damage;

    void Update()
    {
        if(player == null)
        {
            player = GameObject.FindGameObjectWithTag("Player").GetComponent<PlayerController2D>();
        }
    }

    void OnTriggerEnter2D(Collider2D col)
    {

        if (col.CompareTag("Player"))
        {
            player.Damage(damage);
        }
    }
}